Mode locked laser diode in a high power solid state regenerative amplifier and mount mechanism

ABSTRACT

A mode locked as a seed source for a solid state regenerative amplifier system is disclosed. The system includes components for forming an external cavity laser with a semiconductor amplifier, exciting and mode locking the cavity laser to emit optical pulses with a linearly time varying optical frequency, collecting and collimating the optical pulses, isolating the optical pulses and amplifying the optical pulses for a selected application. The selected applications include but are not limited to medical imaging, fuel diagnostics, ultrafast spectroscopic measurements, network synchronization, distributed optical clock network, electro-optic sampling, timing Jitter reduction, a source for inducing nonlinear optical effects, and optical time domain relectometry. A mount mechanism support for an optic system is also disclosed. The mount support includes an optic component such as a semiconductor laser diode, a semiconductor optical amplifier, and a fiber optical amplifier as well as mounts for the optic component. The mount further includes a stud for supporting the optic component, cooling and heat-sinking elements for the component, and an isolator for thermally isolating and separating the mounts from the elements. The thermal isolator includes material selected from teflon and double-panel glass. The mounts can further include a vertical mounting block with one side attached to the isolator and a second mounting block positioned perpendicular to and supporting the vertical mounting block.

This is a divisional of application Ser. No. 08/236,373 filed May 2,1994, now U.S. Pat. No. 5,469,454.

BACKGROUND AND PRIOR ART

The need exists for a compact, efficient high power ultrafast opticalsources for applications in science and engineering fields. Presently,the prevalent ultrafast laser system is the mode locked TitaniumSapphire laser. This laser system requires a large frame Argon ion laserwhich requires a physical size space of approximately forty(40) squarefeet. The prior art system further requires cooling water andsubstantial electrical current which can be impractical for most usefulapplications. Cooling water requirements are 30 gallons per minute @ 65degrees F. The electrical requirements include a three phase powersupply generating 30 Amps. The prior art systems has poor reliability,and poor maintenance records. Furthermore, the prior art mode lockedTitanium Sapphire laser is expensive having a total cost ofapproximately $150,000.

Thus, a laser system is needed that is capable of operating with astandard electrical wallplug that uses 110 volts of alternating currentand should be operable with only one on-off switch. The desired lasersystem should only occupy a space smaller than a few square feet. Thesystem needed should be reliable in that they should operate for periodsof five years without failure. Overall efficiency of laser systemsshould be near 50 percent. The materials used in the laser system shouldbe cheap, efficient, reliable, readily available and non hazardous. Theprior art Argon ion pumped mode locked Titanium Sapphire laser systemdoes not meet these needs.

Prior art laser mounts have poor designs that cause many problems suchas but not limited to heat transfer between the components which hastensfatigue and shortens the life span of the various components that makeup the laser system. For example, as the laser components heat up duringoperation, there is usually a heat transfer to the mounts for the laser.

Thus, the need exists for an improved mount mechanism for laser systems.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide a compactmode locked laser as a seed source for a solid state regenerativeamplifier system.

The second object of this invention is to provide an efficient modelocked laser as a seed source for a solid state regenerative amplifiersystem.

The third object of this invention is to provide a unique self startingmechanism for nonlinearly mode locked laser systems.

The fourth object of this invention is to provide a self sustainingmechanism for nonlinearly mode locked laser systems.

The fifth object of this invention is to provide a method of allowingthe synchronization and triggering of solid state amplifier systems inconjunction with the mode locked diode laser.

The sixth object of the invention is to provide a prechirped pulse asthe seeded optical pulse in a solid state regenerative amplifier system.

The seventh object of the invention is to provide a mounting structurefor a laser to isolate mechanical movements owing to coolingrequirements.

A preferred mode locked as a seed source for a solid state regenerativeamplifier system comprises means for forming an external cavity laserwith a semiconductor amplifier, exciting and mode locking the cavitylaser to emit optical pulses with a linearly time varying opticalfrequency, collecting and collimating the optical pulses, isolating theoptical pulses and amplifying the optical pulses for a selectedapplication. The selected applications include but are not limited tomedical imaging, fuel diagnostics, ultrafast spectroscopic measurements,network synchronization, distributed optical clock network,electro-optic sampling, timing Jitter reduction, a source for inducingnonlinear optical effects, and optical time domain relectometry.

A preferred mount mechanism support for an optic system comprises anoptic component such as a semiconductor laser diode, a semiconductoroptical amplifier, and a fiber optical amplifier as well as mounts forthe optic component. The mount further includes a stud for supportingthe optic component, cooling and heat-sinking elements for thecomponent, and an isolator for thermally isolating and separating themounts from the elements. The thermal isolator includes materialselected from teflon and double-panel glass. The mounts can furtherinclude a vertical mounting block with one side attached to the isolatorand a second mounting block positioned perpendicular to and supportingthe vertical mounting block.

Further objects and advantages of this invention will be apparent fromthe following detailed description of a presently preferred embodimentwhich is illustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a mode locked diode laser in a solid state regenerativeamplifier system of FIG. 1.

FIG. 2 shows an overall layout of the mode locked diode laser systeminvention.

FIG. 3 shows a graph of the generated output optical pulse from thediode laser, which plots the second harmonic intensity autocorrelationfunction verses time delay.

FIG. 4 shows a simplified schematic of the diode laser system of FIG. 1,including the generation and amplification of a chirped pulse withsubsequent pulse compression.

FIG. 5 shows a side view of the laser mount.

FIG. 6 shows a view of the laser mount of FIG. 5 along arrow A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

THE MODE LOCKED DIODE LASER IN A SOLID STATE REGENERATIVE SYSTEM

The invention disclosed has several key features over the prior art. Thesubject invention is compact, efficient, has low maintenance, electricalsychronization, low timing jitter, 110 VAC operation, provides apre-chirped pulse, is bandgap engineerable, is further completelyelectrically pumped and can be potentially integrable.

The subject invention is compact since the laser system is able to befit within limited space such as four (4) square feet, instead of theforty (40) square feet required by the prior art Titanium Saphire laser.The subject invention is efficient since the laser system can beoperated with 110 VAC from a standard wallplug and thus without the needof excessive power requirements.

The subject invention requires low maintenance which allows the lasersystem to be contained within a closed permanent structure such ascomputers and within satellites. This laser system has electricalsynchronization capability which facilitates the operation of the laser.The invention has low timing jitter which further facilitatessynchronization.

This laser system provides a pre-chirped pulse that avoids the use of apulse stretcher in standard chirped pulse regenerative amplifierschemes. The invention is bandgap engineerable which allows a user togenerate any desirable wavelength from the laser without the need ofrelying on naturally ocurring lasting transitions in condensed matter.

The subject laser system is completely electrically pumped where an allsolid state oscillator allows for mechanical robustness and enhancedreliability. The subject laser system can be integrable on chip and withoptical fiber media which allows for direct fabrication withsemiconductor electronics and can be manufactured in packages with sizeson the order of a computer disc.

FIG. 1 shows a mode locked diode laser in a solid state regenerativeamplifier system. This design eliminates the need for an expensive$120,000 to $150,000 Argon ion pump laser and mode-locked TitaniumSaphire laser oscillator. The components of the system of FIG. 1 are nowdefined.

Referring to FIG. 1, M1 through M7 are mirrors. Element 102 is asemiconductor multiple quantum well saturable absorber on a mirrorsurface structure such as a metal mirror or a dielectric mirror. 110 isan output coupler mirror that can be typically broadband and dielectricwith a reflectivity of approximately between 1 and 99%. Elements 124,126 and 132 are standard dielectric type high reflector mirrors that areused to steer the optical beam. Elements 142 and 148 are high powernarrowband dielectric mirrors that help reduce amplified spontaneousemission.

Referring to FIG. 1, L1 to L4 are lens. Elements 104, 108, 114 and 118are focussing/collimating lenses with large numerical apertures. 106 isthe SOA which is a diode such as semiconductor optical amplifier,GaAs/AlGaAs gain guide, thin active region double heterostructure, highpower superluminescent laser diode. 112 refers to I which is an opticalisolator that can be made from Yttrium Indium Garnet(YIG) placed withina magnetic field that can be designed to operate between 700 nanometersand 900 nanometers. 120 and 122 refer to G1 and G2 respectively whichare diffractive grating with 1800 line pairs per millimeter with a goldreflective coatings. 128 and 138 refer to PBS₁ and PBS₂ respectively,which are high power thin film dielectric polarizing beam splitters thatcan be designed to operate between 700 nm and 900 nm. Output 130 goes toany application or experiment requiring the output light pulsesgenerated from this invention. 134 refers to FR which is a FaradayRotator which rotates the plane of polarization depending on whichdirection the light is traveling. 136 is a wave plate of size λ/2 whichrotates the plane of polarization of the light passing through thedevice. 140 refers to a pockel cell which is an electro-optic crystalthat rotates the plane of polarization of light passing through thedevice when an electric field is applied. 144 refers to an Nd:YAG solidstate laser or similar type laser which pumps or excites the solid stateregenerative amplifier medium. 146 refers to the solid stateregenerative amplifier gain medium such as Titanium Sapphire: Ti: Al₂O₃.

The operation of FIG. 1 will now be discussed. The SOA 106 is placedwithin an optical resonator thus making an external cavity laser. TheSOA/external cavity 106 is excited with DC current 103 and actively modelocked with RF current 109. The SOA/external cavity laser 106 is alsopassively mode locked by saturable absorber 102. Prechirped opticalpulses are transmitted through mirror M2 110 and passed through theoptical isolator 112. The optical pulses are amplified by a second SOA116. These pulses can be additionally stretched or compressed if desiredby grating pair G1, 120 and G2, 112. The optical pulses are directedtoward the solid state regenerative amplifier with turning mirrors M3,124 and M4, 126. The light is appropriately polarized from the lasersuch that it is transmitted through PBS1, 128 and injected into theFaraday rotator 134. No optical pulses pass through the hall wave plate136 and the plane of polarization is rotated.

Again referring to FIG. 1, the optical pulses are reflected into theregenerative amplifier cavity M6, 142 and M7, 148, by PBS2, 138. Theplane of polarization is rotated by the Pockels cell 140, after a doublepass so that the light will not be reflected by PBS2, 138. The opticalpulses are amplified in a solid state gain media(e.g. Ti: Al₂ O₃), 146,which is excited by another pump laser 144. The optical pulses arereflected by M7, 148 returning the optical pulse into the solid stategain media 146 for additional amplification. An electric field isapplied to the Pockels cell 140, which alters the birefringence, so thatno polarization rotation is experienced. This keeps the optical pulsestrapped in the regenerative amplifier cavity. The optical pulsescontinue to bounce back and forth between mirrors M6 142, and M7 148,and continue to be amplified by the gain media 146. After several roundtrips, the electric field on the Pockels cell is removed, which allowsthe plane of polarization of the light pulses to be rotated. The lightpulses are then reflected out of the regenerative cavity by PBS2 138.The polarization is rotated again by the half wave plate 136 and alsorotated once more by the Faraday rotator 134. The optical pulses arereflected towards PBS1 128, by M5 132. The optical pulses are nowreflected out of the system by PBS1 128. The resultant pulses areultrashort, high power optical pulses at output 130 FIG. 1, which can bedirected to an application, measurement, diagnostic equipment and thelike, not shown.

FIG. 2 shows a schematic of the mode locked semiconductor laser systemof FIG. 1. Time, t is 206 fs and P≧165 W, λ is 838 nm and fm is 335 MHz.The components of FIG. 2 will now be described. 202 refers to asemiconductor multiple quantum well saturable absorber in contact with ahigh reflector mirror, which is similar to 102 of FIG. 1. Component 204is a focusing/collimating lense. 206 is a beam splitter. 208 is afocusing/collimating lens. 210 is semiconductor optical amplifier(SOA)or a travelling wave amplifier. 212 and 214 comprise a bias tee. 216 isa focusing/collimating lens. 218 is an adjustable slit. 220 is a fourprism sequence. 222 is a collimating/focusing lens. 224 is an outputcoupler. 226 is a collimating/focusing lens. 228 is a turning mirror.230 is an isolator. 240 refers to a semiconductor optical amplifier. 242is an optional stretched/compressed amplified optical pulses which goesto diagnostics as an autocorrelator, photodetector, spectrometer,experiment, application, and the like. 244 are the stretched amplifiedoutput optical pulses which goes to diagnostics as an autocorrelator,photodetector, spectrometer, experiment, application, and the like. 250and 260 are diffraction gratings. 252 is a reflecting mirror. 254 and258 are lenses in a telescope configuration. 256 is an adjustable slitsimilar to component 218.

A description of the operation of the components of FIG. 2 will now bedescribed. The SOA 210, is placed in an external optical cavity formedby output coupler 224 and the MQW saturable mirror 202. Both directcurrent DC and radio frequency RF current is supplied through the biastee 212, 214. The light emission from SOA 210 is collected andcollimated by lenses 208 and 216. Light is focussed on MQWabsorber/mirror 202 by lens 204. Light is then directed through slit218, to control the transverse mode profile. Light is directed throughprism sequence P 220 and focussed onto the output coupler 224 by lens222. When appropriately biased with electric current and opticallyaligned, the laser becomes mode locked, emitting optical pulses with alinearly time varying optical frequency (chirped pulse). The opticalpulses are collected and collimated by lens 226 and directed to anoptical isolator 230 by a turning mirror 228. The optical pulses areamplified in SOA 240 and can either be utilized directly at output 244,or directed to an optical dispersion encorporator/compensator 250-260.The output from the dispersion apparatus 242 are high power opticalpulses which can be diagnosed, utilized in measurements, experiments,applications and the like.

FIG. 3 is a plot illustrating the performance of the laser system ofFIG. 1. In FIG. 3, a generated ultrafast optical pulse is shown comparedto the second harmonic intensity autocorrelation function plotted versesthe time delay. This plot shows an optical pulse of 207 femtoseconds induration.

FIG. 4 shows a schematic diagram of what the optical pulse looks like atseveral points in the laser system of FIG. 2. In reference to FIG. 4,402 refers to a compact, efficient mode locked laser such as a modelocked semiconductor laser. 404 refers to a compact, efficient opticalamplifier such as the SOA. Components 406 and 48 refer to mirrors suchas to direct light into an optical temporal dispersion system. 410 is anoptical dispersion system which can temporally expand or compressoptical pulses. And 412 is the resultant high power ultrafast opticalpulses which can be used for measurements, applications, experiments andthe like. The operation of FIG. 4 corresponds to the operation ofcomponents 220 to 260 of FIG. 2.

The laser system described in FIGS. 1 through 4 has application in wideareas such as but not limited to medical imaging, fuel diagnostics,ultrafast spectroscopic measurements, network synchronization,distributed optical clock network, electro-optic sampling, timing Jitterreduction, a source for inducing nonlinear optical effects, and opticaltime domain relectometry.

In medical imaging applications, ultrashort optical pulses can be usedto image structure in optically dense/diffuse media by relying onoptical time of flight techniques. In fuel diagnostic applications,ultrashort high power optical pulses can be used as a tool to measurethe dynamics of electrons, atoms, molecules and other condensed matterparticles on an ultrafast time scale.

In network synchronization applications, a single RF oscillator can beused to drive several mode locked lasers with identical optical cavitylengths. The resultant is several independent, high synchronized opticalpulse trains which can be used as master timing devices in computers,local area networks and the like.

In optical clock distribution applications, the precise timing of thegenerated optical pulse train can be used as analogous ticks of a clockin any system/network/instrument/application which requires a mastertiming signal. The high output power from the laser allows the opticalclocking signal to be split many times thus providing an identicaltiming signal to many independent locations.

In electro-optic sampling applications, the mode locked diode laser isdriven by an RF oscillator. This RF oscillator can also be used totrigger ultrafast electrical signals. Since both the laser and highspeed electrical signals are driven by the same RF oscillator, minimaltiming uncertainties exist between the ultrafast optical and electronicsignals.

In timing jitter reduction, this is a random fluctuation between thetime of arrival of two successive optical pulses in the generatedoptical pulse train. This small timing jitter is the key ingredientwhich allows one to use the laser in clock distribution, networksynchronization and electro-optic sampling.

As a source for inducing nonlinear optical effects, the peak powersachieved by this laser system are sufficient to induce many nonlinearoptical effects. Such effects include but are not limited to SHG (secondharmonic generation), SPM (self-phase modulation), 4 WM (four wavemixing), and TPA (two photon absorption).

In optical time domain relectometry, this technique is basically anoptical radar technique. Here, a short optical pulse is emitted anddirected towards an object. The reflected light from the object iscollected and the amount of time elapsed between the emitted andreflected optical pulse is measured giving information about theposition and location of the object and target.

MOUNT MECHANISM

FIG. 5 shows a side view of the laser mount 500. FIG. 6 shows a view ofthe laser mount 500 of FIG. 5 along arrow A. The components of FIGS. 5and 6 will now be defined. 502 refers to a thermoelectric cooler such asa Melcor thermoelectric cooler, or other Peltier cooling element. 504 isa heat sink for removing heat. 506 is a stud or any other mounting blockfor a semiconductor optical laser or an amplifier. 508 refers to asemiconductor laser diode, semiconductor optical amplifier, fiberoptical amplifier, or any other device which requires operating at atemperature different from the environment and cannot experience anymovement while being cooled by the Peltier cooling element. 509 areelectrical contacts. 510 is a thermal isolation such as any material ordevice which does not conduct heat such as but not limited to teflon anddoubled panel glass. 512 and 514 respectively refer to mounting blockssuch as any material capable of providing sufficient strength andrigidity to support the laser diode, stud, thermoelectric cooler andheat sink such as copper.

Referring to FIGS. 5 and 6, the operation of mounting mechanism 500 willnow be discussed. Mounting structure 500 separates mounting blocks 512and 514 from cooling elements 502 and from heat-sinking elements 504using thermal isolation means 510. This separation thus avoids smallmovements caused by the thermoelectric cooler 502 when in operation.Mounting block 514 supports the entire mounting structure. Mountingblock 512 supports the laser/stud 506, 508, 509, and is thermallyisolated from these components by thermal isolator 510. With thisarrangement there is no heat transfer to the mounting structure aslaser/stud 506, 508, 509, gets heated by current injection throughelectrical contact 509. Cooling element 502 provides a means for keepingthe laser/stud 506, 508, 509, at an appropriate operating temperature.As the cooling element is operated, small expansions and contractionsoccur in the cooling element 502. Cooling element 502 is connected to aheat sink 504 which removes heat from the cooling element 502 and thelaser/stud device 506, 508, 509. Since the heat sink/cooling element 504is floating in a cantilever arrangement and is not directly connected tothe mounting block 512, 514, the small movements which occur in thecooling element 502 are not transferred to the laser/stud device 506,508, 509. Thus, the mechanical stability of the laser/stud device isprovided by the thermal isolator 510 and mounting blocks 512, 514, whilethe pathway for heat transfer is through the thermoelectric cooler 502forming a floating/cantilever heat sink.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

I claim:
 1. A cantilever mount support that prevents expansion andcontraction movements by cooling and heat sinking means from reaching anattached optical laser/amplifier component that is being cooled by thecooling and heat-sinking means, comprising:an optic component, thecomponent chosen from one of a laser optical amplifier, a fiber opticamplifier and an optical laser; a submount having an upper surface, afirst side surface and a second side surface, the upper surface formounting the optic component; a rigid base mount attached to the firstside surface of the submount for rigidly supporting the submount;cooling and heat-sinking means for cooling the optic component, thecooling and heat-sinking means attached to and cantilevered from thesecond side surface of the submount; and a thermo-isolator meansattached between the submount and the vertical mount for eliminatingheat transfer between the optic component and the vertical mount,wherein expansion and contraction movements caused by the cooling andheat-sinking means which would be transferred and passed to the opticcomponent are eliminated.
 2. The cantilever mount support for the opticsystem of claim 1, wherein the submount further includes:a stud forsupporting the optic component.
 3. The cantilever mount support for theoptic system of claim 1, wherein the thermal isolator includes materialselected from at least one of:Teflon and double-panel glass.
 4. Thecantilever mount support for the optic system of claim 1, wherein therigid base mount further includes:a vertical mounting block with anupper side attached to the first side surface of the submount; and asecond mounting block positioned perpendicular to and supporting thevertical mounting block.
 5. The cantilever mount support for the opticsystem of claim 1, wherein the cooling and heat-sinking means furtherinclude:a thermoelectric cooler attached to the second surface of thesubmount for cooling the optic component; and a heat sink elementattached to an opposite side of the thermoelectric cooler.